The largest LOCA considered (2 x 50 mm) is the break of the line between the vessel and the boiler of the pressurizer, Figure XIX-3. At the beginning, the power is removed by the break and by the steam generator. As for blackout, the RRPa reach their full power in one thousand seconds. After a stabilisation at the pressure of the secondary safety valve, the primary pressure reaches the threshold pressure of the safety injection system at 4 000 s.

Steam generator tube rupture

During the first thousands seconds, the residual power is removed by the SG and by the RRP system. To prevent steam release to the atmosphere, the steam is condensed in a dedicated pool. With 4 RRP loops and 5 tubes ruptured, the mass of released steam is about 40 to 50 tons according to this heat sink of the RRP; with 8 RRPa, the released steam is 20 tons. After six thousand seconds, the RRP system is enough to cool the reactor and the released steam by SG is stopped.

Conclusion of the design basis accident management

The intrinsic behaviour of SCOR is considerably improved comparing a standard PWR. This is due to the large thermal inertia, the elimination of the large LOCAs and the suppression of soluble boron. Without any action (i. e. no RRP and no safety injection), the delay before core dewatering is one and half-hour after the beginning of the most penalizing transient.

Safety calculations showed that all the transients were correctly managed in a passive way (in the vessel, in the RRP loop, and in the heat sink) with only 4 out of 16 RRP, whatever the heat sink: pool or air-cooling tower. This represents a redundancy of 16 times 25%. The RRP operation is compatible with an active or passive way whatever the primary pressure or the temperature. The in-vessel heat exchangers of the RRP loop being located very close to the core, and thanks to the flow bypass of the venturi, the RRPs are operational in two-phase flow mode in case of small primary water inventory. Long term cooling may be ensured in total passive condition thanks to the RRP with air-cooling tower. Only a safety injection at 20 bars with a small flow rate is needed one hour after the beginning of the most penalizing LOCA, that is a double break of the pressurizer line (2 x 50 mm). In the event of SGTR, the steam released from the safety valves of the secondary circuit is condensed in a dedicated pool. There’s no released steam in the atmosphere.

The comparison of the typical design basis events between standard PWRs and SCOR is summarized in Table XIX-1.